The wedding party included Prince George, a page boy at 3, and 2-year-old Princess Charlotte, a bridesmaid. She even become a contributor for Vanity Fair.
Prince Harry and his girlfriend Meghan Merkle will also be in attendance.
Just as Pippa was Maid of Honour at her sister Kate's 2011 royal wedding, the Duchess of Cambridge will return the favour.
Swiss tennis player Roger Federer and his wife Mirka arrive at St Mark's Church in Englefield, England, ahead of the wedding of Pippa Middleton and James Matthews, Saturday, May 20, 2017. She arrived in a open-top vehicle accompanied by her father, smiling broadly and waving to crowds gathered near St. Mark's church in Englefield, about 50 miles west of London.
The Duchess of Cambridge wore a peach dress and matching hat and flashed a smile at the cameras while helping the children in the bridal party go into the church.
Kate Middleton is getting ready for her little sister's wedding this weekend, but she looks as calm and poised as ever.
The details of Miss Middleton's wedding gown have also been a tightly held secret, with the names of a number of designers being put forward by royal watchers, from Amanda Wakeley and Jenny Packham to Oscar De La Renta, Giles Deacon and Alexander McQueen.
The heavy downpour that started minutes after the beginning of the service ended a few moments before the church doors opened.
It's set to be Britain's wedding of the year.
Before the wedding, a crowd of well-wishers and reporters gathered outside the church grounds on a day of sporadic rain.
Matthews was in high spirits as he waited outside the church with his brother and best man Spencer Matthews and James Middleton. Do you think Kate Middleton will be in Pippa's wedding party?
He proposed to Pippa during a trip to the Lake District in June 2016 after they had only been dating for a year - though they have been friends for more than 10 years and were briefly in a relationship in 2012.
The first royal to arrive at the venue was Princess Eugenie, who opted for a navy dress and a simple cream headpiece.
The church is conveniently very close to the Middleton estate in Bucklebury, where the reception will take place.
Sky News is reporting that the Giles Deacon wedding dress could have cost up to 40,000 pounds.
Kate, Duchess of Cambridge, right, stands with her son Prince George as she looks across at Pippa Middleton and James Matthews after their wedding. She will no doubt draw comparisons to the 2011 Royal Wedding with her cathedral-length veil.
The groom, brother of former Made In Chelsea regular and reality television personality Spencer Matthews, arrived shortly before the bride.
Despite that, Pippa's parents Michael and Carole are expected to foot the wedding bill, which, is expected to exceed $500,000.